1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus, and more particularly to a photoelectric conversion apparatus that outputs digital signals.
2. Description of the Related Art
Photoelectric conversion apparatuses used in digital cameras and the like are required to have various functions.
In Japanese Patent Laid-Open No. 2008-283331, a technique is described in which the number of channels is changed in accordance with the operation mode. In this publication, a method is described in which the number of channels between an image pickup unit including a sensor portion, which is a semiconductor device, and an image processing unit. Signals are output from the sensor portion using a certain number of channels regardless of the operation mode. The channels are associated with pixels in every multiple columns. A row of pixels in the sensor portion is simultaneously selected and then signals from pixels in a plurality of columns that are adjacent to one another are transmitted to a subsequent stage through the certain number of channels.
In Japanese Patent Laid-Open No. 2003-179816, a configuration is disclosed in which a pixel portion is divided into a plurality of regions using a plurality of columns that are adjacent to one another as a unit, thereby realizing a high-speed reading by providing an output port for each region. If signals are to be read from a particular region of the pixel portion, all the signals are read from a single output port.
However, in the configuration disclosed in Japanese Patent Laid-Open No. 2003-283331, since the channels in the sensor portion are associated with the pixels in every multiple columns, a signal line that extends over the entirety of the sensor portion is needed. Therefore, the parasitic capacitance and the wiring resistance of the signal line become undesirably large, thereby requiring large power consumption for driving this large load. Furthermore, because settling time of signals becomes longer as load on the signal line becomes larger, it is difficult to realize high-speed reading.
In addition, in the configuration disclosed in Japanese Patent Laid-Open No. 2003-179816, analog signals are transmitted to each output port. Analog signals are attenuated by the capacitance of a signal line. Therefore, in the configuration disclosed in Japanese Patent Laid-Open No. 2003-179816, variation in the signal level of the signals obtained from the output ports is caused if there is variation in the capacitance of a signal line between regions, thereby deteriorating the quality of an obtained image.
Furthermore, in the configuration disclosed in Japanese Patent Laid-Open No. 2003-179816, signals are obtained from a single output port during random access. Therefore, it is difficult to realize high-speed reading.